Ukraine’s president, Petro Poroshenko, has appealed for the Trump administration to maintain sanctions against Russia, saying it is the only way to keep Vladimir Putin at the negotiating table and Russian tanks away from central Europe.

He said continued support for Ukraine was a test of whether the west was in decline, as Moscow has asserted, or had the will to fight for its values.

“Sanctions should stay firm on Russia,” Poroshenko said. “Don’t believe those who think that sanctions bring nothing. It is the only mechanism to keep Russia at the negotiating table. If it was not for sanctions and a newly born Ukrainian army, Russian tanks would be standing much deeper in Europe.

“Putin started his campaign in Ukraine with a sense of impunity. Because of sanctions he has a sense of hesitation.”

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Ukraine is bogged down in a three-year conflict with pro-Russia separatists in eastern Ukraine that followed Moscow’s annexation of Crimea. It is awaiting a preliminary judgment from the international criminal court on its claim that Russia has acted illegally by invading Ukraine.

Poroshenko, speaking at the Chatham House thinktank in London on Tuesday, said Russia was not only rejecting the world order but “trying to build an alternative reality based on alternative values”.

He said he was reassured that the new Trump administration was now firm in its support for Ukraine in its battle to prevent Russian aggression, saying he had had a great conversation last week with the US secretary of state, Rex Tillerson.

Ukraine had been concerned that the US would abandon or loosen sanctions in an attempt to seal a new relationship with Putin, but Poroshenko, sounding his most optimistic note so far on US intentions, suggested the Trump administration understood the conflict did not represent a clash of civilisations but “a clash between a world of rules and a world of armed force”.

“Politically and diplomatically America has a clear view now on Ukraine’s role, and Russian propaganda claims,” he said.

Tillerson had recently been quoted asking why US taxpayers should back sanctions against Russia, but the US state department insists it was simply a probing question.

Poroshenko is in the UK for talks with the prime minister, Theresa May, and the foreign secretary, Boris Johnson, including on possible further technical military support for his government.

The UK also seems to be increasingly convinced that the Trump administration no longer intends to strike up a new bilateral relationship with Russia, and if anything will be more interventionist in Syria against its Moscow-supported president, Bashar al-Assad.

Johnson, speaking in the Commons earlier on Tuesday, claimed Trump’s military assault on Syrian warplanes a fortnight ago was an “emphatic message that the era during which Assad’s barbarism met with passivity and inaction has finally come to an end”.

Johnson has not ruled out further sanctions being imposed on individual Russian military officers if a UN fact-finding mission shows they were complicit in the recent chemical attacks in Syria. He said it was “a media ectoplasm” to suggest he had ever advocated new general economic sanctions against Russia.

Poroshenko couched his appeal for US support in terms of maintaining world backing for nuclear non-proliferation, pointing out that Ukraine had abandoned a massive arsenal of nuclear weapons in 1994 in return for western and Chinese guarantees to Ukraine on its sovereignty and borders.

He asked that if the commitments given by five nations to protect Ukraine under the Budapest agreement were abandoned, “what lessons would be drawn by other nations contemplating going nuclear?”